U.S. patent number 7,141,807 [Application Number 10/971,173] was granted by the patent office on 2006-11-28 for nanowire capillaries for mass spectrometry.
This patent grant is currently assigned to Agilent Technologies, Inc.. Invention is credited to Timothy H. Joyce, Jennifer Qing Lu.
United States Patent |
7,141,807 |
Joyce , et al. |
November 28, 2006 |
Nanowire capillaries for mass spectrometry
Abstract
A capillary for a mass spectrometry system is described. The
capillary comprises a channel and a tip, and at least one of the
channel and the tip comprises a nanowire material.
Inventors: |
Joyce; Timothy H. (Mountain
View, CA), Lu; Jennifer Qing (Milpitas, CA) |
Assignee: |
Agilent Technologies, Inc.
(Santa Clara, CA)
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Family
ID: |
36315354 |
Appl.
No.: |
10/971,173 |
Filed: |
October 22, 2004 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20060097145 A1 |
May 11, 2006 |
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Current U.S.
Class: |
250/492.1;
257/14; 257/9; 257/30; 257/28; 257/10; 438/105; 438/3; 250/284 |
Current CPC
Class: |
H01J
49/04 (20130101); H01J 49/167 (20130101) |
Current International
Class: |
B01D
59/44 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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WO 2005088293 |
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Sep 2005 |
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WO |
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Primary Examiner: Berman; Jack
Assistant Examiner: Hashmi; Zia R.
Claims
We claim:
1. A mass spectrometry capillary, comprising: a channel and a tip,
at least one of the channel and the tip comprising a nanowire
material wherein the mass spectrometry capillary is configured to
produce ions from analytes passing through the mass spectrometry
capillary.
2. The mass spectrometry capillary of claim 1, wherein the tip
comprises a coating that comprises the nanowire material.
3. The mass spectrometry capillary of claim 1, wherein the nanowire
material provides a hydrophobic surface, and the hydrophobic
surface exhibits a contact angle with respect to water that is
greater than 100.degree..
4. The mass spectrometry capillary of claim 3, wherein the contact
angle is greater than 105.degree..
5. The mass spectrometry capillary of claim 1, wherein the nanowire
material comprises a resistivity that is less than 0.1
.OMEGA.cm.
6. The mass spectrometry capillary of claim 5, wherein the
resistivity is less than 0.01 .OMEGA.cm.
7. An ion source for a mass spectrometry system, comprising: a
capillary configured to produce a spray of ions and comprising a
set of nanowires that are positioned with respect to one another to
define an internal passageway.
8. The ion source of claim 7, wherein each of the set of nanowires
provides a hydrophobic surface to regulate the spray of ions.
9. The ion source of claim 7, wherein at least one of the set of
nanowires comprises a material selected from the group consisting
of carbon, silicon, silicon oxide, germanium, gallium nitride, zinc
oxide, zinc selenide, and cadmium sulfide.
10. The ion source of claim 7, wherein at least one of the set of
nanowires comprises a material selected from the group consisting
of chromium, tungsten, iron, gold, nickel, titanium, and
molybdenum.
11. The ion source of claim 7, wherein the capillary comprises a
tip that comprises the set of nanowires.
12. The ion source of claim 7, further comprising: an electrode
positioned adjacent to the capillary, the electrode comprising an
aperture configured to receive the spray of ions when a voltage
between the capillary and the electrode is applied.
13. The ion source of claim 12, further comprising: a feedback loop
in electrical connection with the capillary and the electrode, the
feedback loop being configured to regulate the spray of ions by
adjusting the voltage between the capillary and the electrode.
14. An ion source for a mass spectrometry system, comprising: a
capillary comprising a tip that comprises a nanowire composite
material; an electrode positioned adjacent to the capillary; and a
power source in electrical connection with the capillary and the
electrode, the power source being configured to apply a voltage
between the capillary and the electrode.
15. The ion source of claim 14, wherein the nanowire composite
material is substantially ordered.
16. The ion source of claim 14, wherein the tip comprises a coating
that comprises the nanowire composite material.
17. The ion source of claim 14, wherein the tip comprises a
hydrophobic surface that comprises the nanowire composite material,
and the hydrophobic surface exhibits a contact angle with respect
to water that is greater than 100.degree..
18. The ion source of claim 14, wherein the nanowire composite
material comprises a resistivity that is less than 0.1
.OMEGA.cm.
19. A mass spectrometry system, comprising: (a) an ion source
comprising: (i) a capillary configured to pass a sample stream
comprising analytes, the capillary comprising a portion that is
exposed to the sample stream when the sample stream passes through
the capillary, the portion of the capillary comprising a nanowire
material; and (ii) an electrode positioned with respect to the
capillary, wherein, when a voltage between the capillary and the
electrode is applied, ions are produced from the analytes and are
directed towards the electrode; and (b) a detector positioned with
respect to the ion source to detect the ions as a function of mass
and charge.
20. The mass spectrometry system of claim 19, wherein the nanowire
material comprises a nanowire composite material.
21. The mass spectrometry system of claim 20, wherein the nanowire
composite material comprises a matrix material and a set of
nanowires dispersed in the matrix material.
22. The mass spectrometry system of claim 21, wherein the matrix
material is selected from the group consisting of ceramics,
glasses, metals, metal oxides, alloys, and polymers.
23. The mass spectrometry system of claim 19, wherein the ion
source further comprises: a feedback loop electrically connected to
the capillary and to the electrode, the feedback loop being
configured to regulate the voltage between the capillary and the
electrode.
Description
TECHNICAL FIELD
The technical field of the invention relates to analytical
instruments and, in particular, to mass spectrometry.
BACKGROUND
Various analytical instruments can be used for analyzing proteins
and other biomolecules. More recently, mass spectrometry has gained
prominence because of its ability to handle a wide variety of
biomolecules with high sensitivity and rapid throughput. A variety
of ion sources have been developed for use in mass spectrometry.
Many of these ion sources comprise some type of mechanism that
produces charged species through spraying. One particular type of
technique that is often used is Electrospray Ionization ("ESI").
One benefit of ESI is its ability to produce charged species from a
wide variety of biomolecules such as proteins. Another benefit of
ESI is that it can be readily used in conjunction with a wide
variety of chemical separation techniques, such as High Performance
Liquid Chromatography ("HPLC"). For example, ESI is often used in
conjunction with HPLC for identifying proteins.
Typically, ESI produces a spray of ions in a gaseous phase from a
sample stream that is initially in a liquid phase. For a
conventional ESI mass spectrometry system, a sample stream is
pumped through a metal capillary, while a relatively high electric
field is applied between a tip of the metal capillary and an
electrode that is positioned adjacent to the tip of the metal
capillary. As the sample stream exits the tip of the metal
capillary, surface charges are produced in the sample stream, thus
pulling the sample stream towards the electrode. As the sample
stream enters the high electric field, a combined
electro-hydrodynamic force on the sample stream is balanced by its
surface tension, thus producing a "Taylor cone." Typically, the
Taylor cone has a base positioned near the tip of the metal
capillary and extends up to a certain distance away from the tip of
the metal capillary, beyond which a spray of droplets is produced.
As these droplets move towards the electrode, coulombic repulsive
forces and desolvation lead to the formation of a spray of ions in
a gaseous phase.
During operation of a conventional ESI mass spectrometry system,
characteristics of a Taylor cone can affect characteristics of a
spray of ions, which, in turn, can affect results of mass
spectrometric analysis. Accordingly, it is desirable to produce
Taylor cones with certain reproducible characteristics, such that
results of mass spectrometric analysis have a desired level of
accuracy and reproducibility.
SUMMARY
The invention provides a mass spectrometry system. The mass
spectrometry system comprises an ion source comprising a capillary
configured to pass a sample stream. The capillary comprises a
portion that is exposed to the sample stream when the sample stream
passes through the capillary, and the portion of the capillary
comprises a nanowire material. The ion source also comprises an
electrode positioned with respect to the capillary, wherein, when a
voltage between the capillary and the electrode is applied, ions
are produced from the sample stream and are directed towards the
electrode. The mass spectrometry system also comprises a detector
positioned with respect to the ion source to detect the ions.
The invention also provides an ion source for a mass spectrometry
system. The ion source comprises a capillary configured to produce
a spray of ions and comprising a nanowire material that is
hydrophobic.
In another embodiment, the ion source comprises a capillary
comprising a tip that comprises a nanowire composite material. The
ion source also comprises an electrode positioned adjacent to the
capillary. The ion source further comprises a power source in
electrical connection with the capillary and the electrode, and the
power source is configured to apply a voltage between the capillary
and the electrode.
The invention further provides a capillary for a mass spectrometry
system. The capillary comprises a channel and a tip, and at least
one of the channel and the tip comprises a nanowire material.
Advantageously, embodiments of the invention allow Taylor cones to
be produced with certain reproducible characteristics, such that
results of mass spectrometric analysis have a desired level of
accuracy and reproducibility. For some embodiments of the
invention, reproducibility of Taylor cones can be achieved by using
certain materials that are highly hydrophobic, highly electrically
conductive, highly robust, and highly inert with respect to typical
analytes.
Other aspects and embodiments of the invention are also
contemplated. The foregoing summary and the following detailed
description are not meant to restrict the invention to any
particular embodiment but are merely meant to describe some
embodiments of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the nature and objects of some
embodiments of the invention, reference should be made to the
following detailed description taken in conjunction with the
accompanying drawings.
FIG. 1 illustrates a mass spectrometry system implemented in
accordance with an embodiment of the invention.
FIG. 2 illustrates an electrospray capillary comprising a set of
nanowires, according to an embodiment of the invention.
FIG. 3 illustrates an ion source comprising a feedback controller,
according to an embodiment of the invention.
DETAILED DESCRIPTION
Definitions
The following definitions apply to some of the elements described
with respect to some embodiments of the invention. These
definitions may likewise be expanded upon herein.
As used herein, the singular terms "a," "an," and "the" comprise
plural referents unless the context clearly dictates otherwise.
As used herein, the term "set" refers to a collection of one or
more elements. Thus, for example, a set of nanowires can comprise a
single nanowire or multiple nanowires. Elements of a set can also
be referred to as members of the set. Elements of a set can be the
same or different. In some instances, elements of a set can share
one or more common characteristics.
As used herein with reference to a sample stream, the term
"exposed" refers to being subject to possible interaction with the
sample stream. A material can be exposed to a sample stream without
being in actual or direct contact with the sample stream. Also, a
material can be exposed to a sample stream if the material is
subject to possible interaction with a spray of droplets or a spray
of ions produced from the sample stream in accordance with an
ionization process.
As used herein, the terms "hydrophilic" and "hydrophilicity" refer
to an affinity for water, while the terms "hydrophobic" and
"hydrophobicity" refer to a lack of affinity for water. Hydrophobic
materials typically correspond to those materials to which water
has little or no tendency to adhere. As such, water on a surface of
a hydrophobic material tends to bead up. One measure of
hydrophobicity of a material is a contact angle between a surface
of the material and a line tangent to a drop of water at a point of
contact with the surface. Typically, the material is considered to
be hydrophobic if the contact angle is greater than 90.degree..
As used herein, the terms "electrically conductive" and "electrical
conductivity" refer to an ability to transport an electric current.
Electrically conductive materials typically correspond to those
materials that exhibit little or no opposition to flow of an
electric current. One measure of electrical conductivity of a
material is its resistivity expressed in ohm.centimeter
(".OMEGA.cm"). Typically, the material is considered to be
electrically conductive if its resistivity is less than 0.1
.OMEGA.cm. The resistivity of a material can sometimes vary with
temperature. Thus, unless otherwise specified, the resistivity of a
material is defined at room temperature.
As used herein, the terms "robust" and "robustness" refer to a
mechanical hardness or strength. Robust materials typically
correspond to those materials that exhibit little or no tendency to
fragment under typical operating conditions, such as typical
operating conditions of the electrospray capillaries described
herein. One measure of robustness of a material is its Vicker
microhardness expressed in kg/mm. Typically, the material is
considered to be robust if its Vicker microhardness is greater than
1,000 kilogram/millimeter ("kg/mm").
As used herein, the terms "inert" and "inertness" refer to a lack
of interaction. Inert materials typically correspond to those
materials that exhibit little or no tendency to interact with a
sample stream under typical operating conditions, such as typical
operating conditions of the electrospray capillaries described
herein. Typically, inert materials also exhibit little or no
tendency to interact with a spray of droplets or a spray of ions
produced from a sample stream in accordance with an ionization
process. While a material is sometimes referred to herein as being
inert, it is contemplated that the material can exhibit some
detectable tendency to interact with a sample stream under certain
conditions. One measure of inertness of a material is its chemical
reactivity. Typically, the material is considered to be inert if it
exhibits little or no chemical reactivity with respect to a sample
stream.
As used herein, the term "microstructure" refers to a microscopic
structure of a material and can encompass, for example, a lattice
structure, crystallinity, dislocations, grain boundaries,
constituent atoms, doping level, surface functionalization, and the
like. One example of a microstructure is an elongated structure,
such as comprising a nanowire. Another example of a microstructure
is an array or arrangement of nanowires.
As used herein, the term "nanowire" refers to an elongated
structure. Typically, a nanowire is substantially solid and, thus,
can exhibit characteristics that differ from those of certain
elongated, hollow structures. In some instances, a nanowire can be
represented as comprising a filled cylindrical shape. A nanowire
typically has a cross-sectional diameter from about 0.5 nanometer
("nm") to about 1,000 nm, such as from about 1 nm to about 200 nm,
from about 1 nm to about 100 nm, or from about 1 nm to about 50 nm,
and a length from about 0.1 micrometer (".mu.m") to about 1,000
.mu.m, such as from about 1 .mu.m to about 50 .mu.m or from about 1
.mu.m to about 10 .mu.m. A nanowire can be formed from any of a
wide variety of materials, such as inorganic materials and organic
materials. Examples of nanowires comprise those formed from
semiconductors, such as carbon, silicon, silicon oxide, germanium,
gallium nitride, zinc oxide, zinc selenide, cadmium sulfide, and
the like. Other examples of nanowires comprise those formed from
metals, such as chromium, tungsten, iron, gold, nickel, titanium,
molybdenum, and the like. A nanowire typically comprises a
substantially ordered array or arrangement of atoms and, thus, can
be referred to as being substantially ordered or having a
substantially ordered microstructure. It is contemplated that a
nanowire can comprise a range of defects and can be doped or
surface functionalized. For example, a nanowire can be doped with
metals, such as chromium, tungsten, iron, gold, nickel, titanium,
molybdenum, and the like. It is also contemplated that a nanowire
can comprise a set of heterojunctions or can comprise a core/sheath
structure. For example, a nanowire can comprise a core formed from
silicon and a sheath surrounding the core and formed from silicon
oxide. As another example, a nanowire can comprise a core formed
from zinc oxide and a sheath surrounding the core and formed from
gallium nitride. Nanowires can be formed using any of a wide
variety of techniques, such as arc-discharge, laser ablation,
chemical vapor deposition, epitaxial casting, and the like.
As used herein, the term "nanowire material" refers to a material
that comprises or is formed from a set of nanowires. In some
instances, a nanowire material can comprise a set of nanowires that
are substantially aligned with respect to one another or with
respect to a certain axis, plane, surface, or three-dimensional
shape and, thus, can be referred to as being substantially ordered
or having a substantially ordered microstructure. Alignment of a
set of nanowires can be performed using any of a wide variety of
techniques, such as hybrid pulsed laser deposition/chemical vapor
deposition, microfluidic-assisted alignment, Langmuir-Blodgett
patterning, and the like.
As used herein, the term "composite material" refers to a material
that comprises or is formed from two or more different materials.
In some instances, a composite material can comprise or can be
formed from materials that share one or more common
characteristics. One example of a composite material is one that
comprises or is formed from a nanowire material, namely a nanowire
composite material. A nanowire composite material typically
comprises a matrix material and a set of nanowires dispersed in the
matrix material. Examples of matrix materials comprise ceramics,
glasses, metals, metal oxides, alloys, polymers, and the like.
Additional examples of matrix materials comprise nitrides and
disulfides as, for example, described in the patent of Perkins et
al., "Ionization Chamber for Reactive Samples," U.S. Pat. No.
6,608,318, the disclosure of which is incorporated herein by
reference in its entirety. Further examples of matrix materials
comprise super alloys as, for example, described in the patent of
Perkins, "Super Alloy Ionization Chamber for Reactive Samples,"
U.S. Pat. No. 6,765,215, the disclosure of which is incorporated
herein by reference in its entirety. Composite materials, such as
nanowire composite materials, can be formed using any of a wide
variety of techniques, such as colloidal processing, sol-gel
processing, die casting, in situ polymerization, and the like.
As used herein, the term "ionization efficiency" refers to a ratio
of the number of ions formed in an ionization process and the
number of electrons or photons used in the ionization process.
Attention first turns to FIG. 1, which illustrates a mass
spectrometry system 100 implemented in accordance with an
embodiment of the invention. The mass spectrometry system 100
comprises an ion source 102, which operates to produce ions. In the
illustrated embodiment, the ion source 102 operates to produce ions
using ESI. However, it is contemplated that the ion source 102 can
be implemented to produce ions using any other ionization process.
As illustrated in FIG. 1, the mass spectrometry system 100 also
comprises a detector 106, which is positioned with respect to the
ion source 102 to receive ions. The detector 106 operates to detect
ions as a function of mass and charge.
In the illustrated embodiment, the ion source 102 comprises an
electrospray capillary 108 and an electrode 110, which is
positioned adjacent to the electrospray capillary 108 and serves as
a counter-electrode with respect to the electrospray capillary 108.
The ion source 102 also comprises a power source 112, which is
electrically connected to the electrospray capillary 108 and to the
electrode 110. The power source 112 operates to apply a voltage to
the electrospray capillary 108 and the electrode 110, thus
producing an electric field between the electrospray capillary 108
and the electrode 110. As illustrated in FIG. 1, the ion source 102
also comprises a housing 114, which defines an internal chamber 104
within which the electrospray capillary 108, the electrode 110, and
the power source 112 are positioned.
As illustrated in FIG. 1, the electrospray capillary 108 comprises
a channel 116 and a tip 118. The channel 116 defines an internal
passageway 120 through which a sample stream 122 passes. The sample
stream 122 comprises analytes to be analyzed by the mass
spectrometry system 100. For example, the sample stream 122 can
comprise biomolecules that are dispersed in a suitable solvent,
such as water. In the illustrated embodiment, the positioning of
the electrospray capillary 108 in the vicinity of the electrode 110
at a negative bias produces an electric field gradient at the tip
118 of the electrospray capillary 108. As the sample stream 122
exits the tip 118 of the electrospray capillary 108, a jump in
displacement flux density produces surface charges in the sample
stream 122, which pulls the sample stream 122 towards the electrode
110. In conjunction, a combined electro-hydrodynamic force on the
sample stream 122 is balanced by its surface tension, thus
producing a Taylor cone 124. As illustrated in FIG. 1, the Taylor
cone 124 comprises a base 126 positioned near the tip 118 of the
electrospray capillary 108. The Taylor cone 124 also comprises a
tip 128, which extends into a filament 130. As the filament 130
extends further towards the electrode 110, combined effects of
surface tension, coulombic repulsive forces, and small
perturbations cause the filament 130 to break up and to form a
spray of droplets 132. As these droplets 132 move towards the
electrode 110, coulombic repulsive forces and desolvation lead to
the formation of a spray of ions 134.
As illustrated in FIG. 1, the electrode 110 defines an aperture 136
near its center. The ions 134 pass through the electrode 110 via
the aperture 136 and eventually reach the detector 106. In the
illustrated embodiment, a drying gas 138, such as a nitrogen gas,
flows in a direction counter to the ions 134 to improve ionization
efficiency and to restrain introduction of undesirable materials
into the aperture 136. In the illustrated embodiment, the electrode
110 is positioned in a longitudinal relationship with respect to
the electrospray capillary 108. In other words, an angle defined by
a central axis 140 of the internal passageway 120 and a central
axis 142 of the aperture 136 is substantially at 0.degree..
However, it is contemplated that this angle can be adjusted to
differ from 0.degree., such as from about 75.degree. to about
105.degree.. For example, it is contemplated that the electrode 110
can be positioned in an orthogonal relationship with respect to the
electrospray capillary 108, such that this angle is substantially
at 90.degree..
During operation of the mass spectrometry system 100,
characteristics of the Taylor cone 124 can affect characteristics
of the ions 134 that are produced, which, in turn, can affect
results of mass spectrometric analysis. Accordingly, it is
desirable to produce Taylor cones with certain reproducible
characteristics, such that results of mass spectrometric analysis
have a desired level of accuracy and reproducibility. In the
illustrated embodiment, Taylor cones can be produced with
reproducible characteristics by controlling hydrophobicity of the
electrospray capillary 108. In particular, if the tip 118 of the
electrospray capillary 108 is made sufficiently hydrophobic, the
base 126 of the Taylor cone 124 can be restrained from spreading
along the tip 118 of the electrospray capillary 108. In such
manner, the base 126 of the Taylor cone 124 can be produced with a
reproducible shape and size, which can correspond to a shape and
size of the internal passageway 120 at the tip 118 of the
electrospray capillary 108.
As illustrated in FIG. 1, the tip 118 of the electrospray capillary
108 comprises a hydrophobic material 144. For certain
implementations, the hydrophobic material 144 can form a coating
that at least partly covers one end of the channel 116, which
serves as a substrate. In general, the hydrophobic material 144 can
correspond to any of a wide variety of hydrophobic materials.
Particularly useful hydrophobic materials correspond to those
materials that exhibit a combination of desirable characteristics,
comprising hydrophobicity, electrical conductivity, and robustness.
In terms of hydrophobicity, particularly useful hydrophobic
materials correspond to those materials that provide a hydrophobic
surface, such that Taylor cones can be produced with a reproducible
shape and size. In particular, the hydrophobic surface desirably
exhibits a contact angle with respect to water that is greater than
90.degree., such as greater than about 100.degree., greater than
about 105.degree., or greater than about 110.degree.. In terms of
electrical conductivity, particularly useful hydrophobic materials
correspond to those materials that comprise a relatively low
resistivity, such that an electric field can be properly applied
between the tip 118 of the electrospray capillary 108 and the
electrode 110. In particular, the resistivity is desirably less
than 0.1 .OMEGA.cm, such as less than about 0.01 .OMEGA.cm, less
than about 0.001 .OMEGA.cm, or less than about 0.0001 .OMEGA.cm. As
can be appreciated, use of hydrophobic materials comprising a
relatively low resistivity can avoid the need for an additional
coating of an electrically conductive material, which additional
coating can adversely affect hydrophobicity of the tip 118 of the
electrospray capillary 108. In terms of robustness, particularly
useful hydrophobic materials correspond to those materials that
exhibit little or no tendency to fragment under typical operating
conditions of the electrospray capillary 108, thus increasing
operational lifetime of the electrospray capillary 108. In
particular, the Vicker microhardness of those materials is
desirably greater than 1,000 kg/mm, such as greater than about
2,000 kg/mm, greater than about 2,500 kg/mm, or greater than about
3,000 kg/mm. For example, the Vicker microhardness is desirably
from about 2,500 kg/mm to about 3,500 kg/mm.
It has been discovered that certain materials can be particularly
useful hydrophobic materials, since these materials can exhibit the
combination of desirable characteristics described above. In the
illustrated embodiment, the hydrophobic material 144 desirably
comprises a nanowire material, such as a nanowire composite
material. Advantageously, the nanowire material can exhibit a
higher level of hydrophobicity, a higher level of electrical
conductivity, and a higher level of robustness as compared with
certain other types of hydrophobic materials. A further benefit of
the nanowire material is its higher level of inertness with respect
to typical analytes that can comprise the sample stream 122.
Without wishing to be bound by a particular theory, it is believed
that a substantially ordered microstructure of the nanowire
material contributes to at least some of its desirable and unusual
characteristics.
For certain implementations, the nanowire material can form a
coating, which can be applied using any of a wide variety of
techniques. For example, the nanowire material can be sprayed at
high velocity onto a substrate, such that the nanowire material
mechanically adheres to the substrate. As another example, the
nanowire material can be dispersed in a suitable solvent to form a
"paint," and this paint can be applied to the substrate. In some
instances, the solvent can be relatively inert. However, it is also
contemplated that the solvent can facilitate chemical bonding
between the nanowire material and the substrate. Heat can be
applied to evaporate the solvent or to promote chemical
bonding.
While FIG. 1 illustrates the tip 118 of the electrospray capillary
108 as comprising the hydrophobic material 144, it is contemplated
that other portions of the electrospray capillary 108 can comprise
the hydrophobic material 144. In particular, it is contemplated
that any portion of the electrospray capillary 108 that is exposed
to the sample stream 122 can comprise the hydrophobic material 144.
For example, the channel 116 can also comprise the hydrophobic
material 144, which can form a coating that at least partly covers
a surface surrounding the internal passageway 120. Such
implementation can facilitate a flow of the sample stream 122
through the electrospray capillary 108, which, in turn, can allow
Taylor cones to be produced with reproducible characteristics. In
general, it is contemplated that different portions of the
electrospray capillary 108 can comprise hydrophobic materials that
are the same or different. It is also contemplated that the
electrospray capillary 108 can be substantially formed of the
hydrophobic material 144.
Attention next turns to FIG. 2, which illustrates an electrospray
capillary 208 implemented in accordance with another embodiment of
the invention. The electrospray capillary 208 comprises a channel
216 and a tip 218. In the illustrated embodiment, the electrospray
capillary 208 comprises a set of nanowires, namely nanowires 246,
248, 250, 252, 254, 256, and 258, and the nanowires 246, 248, 250,
252, 254, 256, and 258 are substantially aligned with respect to
one another to form both the channel 216 and the tip 218 of the
electrospray capillary 208. While seven nanowires 246, 248, 250,
252, 254, 256, and 258 are illustrated in FIG. 2, it is
contemplated that more or less nanowires can be used for other
implementations. It is also contemplated that the electrospray
capillary 208 can comprise a separate channel, and the nanowires
246, 248, 250, 252, 254, 256, and 258 can be positioned adjacent to
the separate channel.
As illustrated in FIG. 2, the nanowires 246, 248, 250, 252, 254,
256, and 258 are positioned with respect to one another to define
internal passageways 260, 262, 264, 266, and 268 through which a
sample stream can pass. As the sample stream exits the tip 218 of
the electrospray capillary 208, Taylor cones can be formed at
respective ends of the internal passageways 260, 262, 264, 266, and
268. In the illustrated embodiment, the nanowires 246, 248, 250,
252, 254, 256, and 258 can be positioned with respect to one
another, such that the internal passageways 260, 262, 264, 266, and
268 have sufficient cross-sectional sizes to allow flow of certain
analytes, such as biomolecules.
The electrospray capillary 208 can be formed using any of a wide
variety of techniques. For example, the nanowires 246, 248, 250,
252, 254, 256, and 258 can be positioned with respect to one
another using an Atomic Force Microscope ("AFM"). For certain
implementations, at least a subset of the nanowires 246, 248, 250,
252, 254, 256, and 258 can comprise a core/sheath structure, and
cores of this subset can be at least partly removed using any of a
wide variety of techniques, such as preferential etching with xenon
fluoride. Removal of the cores can define additional internal
passageways through which the sample stream can pass. For such
implementations, it is contemplated that a plug material can be
used to block flow of the sample stream through at least a subset
the internal passageways 260, 262, 264, 266, and 268.
It should be recognized that the embodiments of the invention
described above are provided by way of example, and various other
embodiments are encompassed by the invention. For example, it is
contemplated that the electrospray capillaries described herein can
be advantageously used in conjunction with a control mechanism to
regulate a spray of ions. One example of such a control mechanism
is a feedback controller as described in the co-pending and
co-owned patent application of Sobek, U.S. patent application Ser.
No. 10/896,981, filed Jul. 23, 2004, entitled "Ion Source Frequency
Feedback Device and Method," the disclosure of which is
incorporated herein by reference in its entirety. FIG. 3
illustrates an ion source 302 comprising a feedback controller 358,
in accordance with an embodiment of the invention. The feedback
controller 358 operates to detect a modulation frequency of an
ionization current I(t) between a tip 318 of an electrospray
capillary 308 and a counter-electrode 310, which are positioned in
a module 370. Based on this modulation frequency, the feedback
controller 358 operates to provide feedback regulation of ESI
characteristics by adjusting a voltage V.sub.cc between the tip 318
of the electrospray capillary 308 and the counter-electrode
310.
With reference to FIG. 3, the ionization current I(t) between the
electrospray capillary 308 and the counter-electrode 310 can
experience transient fluctuations in amplitude (i.e., can be
modulated) depending on operating conditions of the ion source 302.
In particular, depending on the voltage V.sub.cc, modulation of the
ionization current I(t) can have characteristics associated with
one of a variety of ESI modes, comprising: (1) a pulsating mode
with lower modulation frequencies ("mode I"); (2) a
constant-amplitude oscillation mode with intermediate modulation
frequencies ("mode II"); and (3) a continuous emission mode with
higher modulation frequencies ("mode III"). Among these three
modes, mode II and mode III typically provide the most desirable
ESI characteristics. Advantageously, a correlation between a
magnitude of the modulation frequency and the different ESI modes
allows the modulation frequency to be used as an indicator of a
particular ESI mode under which the ion source 302 is currently
operating. In turn, the voltage V.sub.cc can be adjusted until the
modulation frequency has a magnitude associated with a desired ESI
mode. However, the modulation frequency typically depends on
characteristics of a Taylor cone that is produced, such as a size
of a base of the Taylor cone. Accordingly, in order for the
modulation frequency to be an accurate and reproducible indicator
of a particular ESI mode, it is desirable to produce Taylor cones
with reproducible characteristics. In the illustrated embodiment,
Taylor cones can be produced with reproducible characteristics by
controlling hydrophobicity of the electrospray capillary 308 in a
similar manner as described above. In particular, if the tip 318 of
the electrospray capillary 308 is made sufficiently hydrophobic,
Taylor cones can be produced with bases of a reproducible shape and
size.
As illustrated in FIG. 3, the feedback controller 358 comprises a
transimpedance amplifier 360, a DC de-coupler 362, a
frequency-to-voltage converter 364, a controller 366, and a
voltage-controlled high-voltage power supply 368. The
transimpedance amplifier 360, the DC de-coupler 362, the
frequency-to-voltage converter 364, the controller 366, and the
voltage-controlled high-voltage power supply 368 comprise a closed
feedback loop to provide feedback regulation of ESI
characteristics.
The transimpedance amplifier 360 operates to convert the ionization
current I(t) into a voltage V(t). Since typical nano-flow
ionization currents can range from about 5 nA to about 150 nA and
can exhibit modulation frequencies up to about 200 kHz, the
transimpedance amplifier 360 desirably has a bandwidth of at least
about 400 kHz and a gain of about 10.sup.7. Amplifiers with such
specifications are commercially available. Alternatively, the
transimpedance amplifier 360 can be implemented using a two-stage
Op-Amp design, such as using a low noise transimpedance module for
current to voltage conversion and a boost Op-Amp stage for further
signal amplification.
The DC de-coupler 362 operates to remove a Direct Current ("DC")
component of the voltage V(t). In turn, the frequency-to-voltage
converter 364 responds to an input frequency of the voltage V(t)
and delivers to the controller 366 an input voltage V.sub.in that
is linearly proportional to this input frequency. In other words,
the transimpedance amplifier 360, the DC de-coupler 362, and the
frequency-to-voltage converter 364 operate to convert frequency
information in the ionization current I(t) into the input voltage
V.sub.in.
In the illustrated embodiment, the controller 366 can be
implemented using a microprocessor 372 that operates to produce an
output voltage V.sub.out from the input voltage V.sub.in in
accordance with a set of processor-executable instructions. The
output voltage V.sub.out controls the voltage-controlled
high-voltage power supply 368, which applies the voltage V.sub.cc
between the tip 318 of the electrospray capillary 308 and the
counter-electrode 310. As illustrated in FIG. 3, the voltage
V.sub.cc is proportional to the output voltage V.sub.out. The
voltage V.sub.cc can be a DC voltage or a DC voltage with an
Alternating Current ("AC") component. For certain implementations,
the DC voltage can be used to establish a highest possible electric
field for which there is no ESI action. High-voltage AC pulses can
be superimposed on the DC voltage to elicit on-demand droplet
formation. The AC pulses can be produced using suitable high
voltage amplifier circuits and can be, for example, sinusoidal,
square-shaped, or triangular-shaped. A shape and a duty cycle of
the AC pulses can be adjusted to control characteristics of a
Taylor cone, thus creating a spray of ions with desired
characteristics. It is contemplated that the AC pulses can be
synchronized with respect to sampling electronics to provide for a
desired level of sensitivity and reproducibility.
In the illustrated embodiment, the voltage V.sub.cc is applied to
the counter-electrode 310, while the transimpedance amplifier 360
is electrically connected to the electrospray capillary 308, which
is grounded. However, it is contemplated that the voltage V.sub.cc
can be applied to the tip 318 of the electrospray capillary 308,
and the ionization current I(t) can be detected at the tip 318 of
the electrospray capillary 308 or at the counter-electrode 310. It
is contemplated that a voltage-controlled flow rate controller can
be used in place of, or in conjunction with, the voltage-controlled
high-voltage power supply 368. The voltage-controlled flow rate
controller can operate to adjust a flow rate of a sample fluid
passing through the electrospray capillary 308 based on the output
voltage V.sub.out.
Desirably, the module 370 is shielded from interfering signals to
improve a signal-to-noise ratio for the operations described above.
Proper shielding can be achieved by, for example, using a grounded
electrically conductive housing. Connections in and out of the
housing can be implemented using coaxial cables.
A practitioner of ordinary skill in the art requires no additional
explanation in developing the electrospray capillaries described
herein but may nevertheless find some helpful guidance by examining
the following articles: Taylor G. I., "Disintegration of Water
Drops in an Electric Field," Proceedings of the Royal Society of
London, A280, 383 397, 1964; Bruins A. P., "Mechanistic Aspects of
Electrospray Ionization," Journal of Chromatography A, 794, 345
347, 1998; Juraschek et al., "Pulsation Phenomena During
Electrospray Ionization," International Journal of Mass
Spectrometry, 177, 1 15, 1998; Cech et al., "Practical Implications
of Some Recent Studies in Electrospray Ionization Fundamentals,"
Mass Spectrometry Reviews, 20, 362 387, 2001; and Lee et al.,
"Taylor Cone Stability and ESI Performance for LC-MS at Low Flow
Rates," Proceedings of the American Society of Mass Spectrometry,
2002; the disclosures of which are incorporated herein by reference
in their entireties.
A practitioner of ordinary skill in the art may also find some
helpful guidance regarding characteristics and formation of
nanowire materials by examining the following articles: Abramson et
al., "Fabrication and Characterization of a Nanowire/Polymer-based
Nanocomposite for a Prototype Thermoelectric Device," J.
Microelectromechanical Systems, 13, 505, 2004; Kuykendall et al.,
"Crystallographic Alignment of High Density Gallium Nitride
Nanowire Arrays," Nature Materials, 3, 528, 2004; Law et al.,
"Semiconductor Nanowires and Nanotubes," Annu. Rev. Mater. Sci.,
34, 83, 2004; Qian et al., "Synthesis of Iron Phosphide
Nanorod/Nanowires in Solution," J. Am. Chem. Soc., 126, 1195, 2004;
Tao et al., "Langmuir-Blodgett Silver Nanowire Monolayers for
Molecular Sensing with High Sensitivity and Specificity," Nano.
Lett., 3, 1229, 2003; Stach et al., "Watching GaN Nanowires Grow,"
Nano. Lett., 3, 867, 2003; Goldberger et al., "Single Crystal
Gallium Nitride Nanotubes," Nature, 422, 599, 2003; Xia et al.,
"Chemistry and Physics of Nanowires," Adv. Mater., 15, 351, 2003;
Gates et al., "Synthesis and Characterization of Crystalline
Ag.sub.2Se Nanowires Through a Template-Engaged Reaction at Room
Temperature," Adv. Func. Mater., 12, 679, 2002; Wu et al.,
"Inorganic Semiconductor Nanowires," Int. J. Nano. (Invited Review,
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ZnO Nanowires and their Optical Properties," Adv. Func. Mater.
(Invited Feature Article), 12, 323, 2002; Zheng et al., "Synthesis
of Ultra-Long and Highly-Oriented Silicon Oxide Nanowires from
Alloy Liquid," Adv. Mater., 14, 122, 2002; Wu et al.,
"Block-by-Block Growth of Si/SiGe Superlattice Nanowires,"
Nanolett., 2, 83, 2002; Wu et al., "Inorganic Semiconductor
Nanowires: Rational Growth, Assemblies and Novel Properties," Euro.
J. Chemistry (Invited Concept Article), 8, 1260, 2002; Song et al.,
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Formation via Chimie Douce Reaction," J. Am. Chem. Soc., 123, 9714,
2001; Kwan et al., "Synthesis and Assembly of BaWO.sub.4 Nanorods,"
Chem. Commun., 5, 447, 2001; Wu et al., "Direct Observation of
Vapor-Liquid-Solid Nanowire Growth," J. Am. Chem. Soc., 123, 3165,
2001; Huang et al., "Catalytic Growth of Zinc Oxide Nanowires
Through Vapor Transport," Adv. Mater., 13(2), 113, 2001; Wu et al.,
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77, 43, 2000; Wu et al., "Germanium Nanowire Growth via Simple
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disclosures of which are incorporated herein by reference in their
entireties.
While the invention has been described with reference to the
specific embodiments thereof, it should be understood by those
skilled in the art that various changes may be made and equivalents
may be substituted without departing from the true spirit and scope
of the invention as defined by the appended claims. In addition,
many modifications may be made to adapt a particular situation,
material, composition of matter, method, process operation or
operations, to the objective, spirit and scope of the invention.
All such modifications are intended to be within the scope of the
claims appended hereto. In particular, while the methods disclosed
herein have been described with reference to particular operations
performed in a particular order, it will be understood that these
operations may be combined, sub-divided, or re-ordered to form an
equivalent method without departing from the teachings of the
invention. Accordingly, unless specifically indicated herein, the
order and grouping of the operations is not a limitation of the
invention.
* * * * *